In assisted ventilation, ventilator cycles are triggered by patient inspiratory efforts. There is no mechanism, however, to insure that ventilator cycles terminate at, or near, the end of inspiratory effort. Because the duration of patient inspiratory efforts (neural TI) varies over a wide range (0.5 to 2.5 seconds), the lack of a link between end of ventilator and patient inspiratory cycles often results in ventilator cycles extending well beyond the inspiratory effort (delayed cycling off) or terminating before the end of inspiratory effort, forcing exhalation when the patient is still trying to inhale. The delayed cycling off in particular is often severe with the ventilator cycle extending throughout the patient's expiratory phase (FIG. 1). Because such delayed cycling off interferes with lung emptying during the patient's expiratory phase, the next breath usually begins before lung volume has returned to the neutral level. This delays ventilator triggering and often causes many patient cycles to be ineffective in triggering the ventilator (ineffective efforts, FIG. 1).
Non-synchrony between patient and ventilator is extremely common. Leung et al found that, on average, 28% of patient's efforts are ineffective (Leung P, Jubran A, Tobin M J (1997), Comparison of assisted ventilator modes on triggering, patient effort, and dyspnea. Am J Respir Crit Care Med 155:1940-1948). Considering that ineffective efforts are the extreme manifestation of non-synchrony, less severe, yet substantial, delays must occur even more frequently. Non-synchrony is believed to cause distress, leading to excessive sedation and sleep disruption, as well as errors is clinical assessment of patients since the respiratory rate of the ventilator can be quite different from that of the patient (e.g. FIG. 1).
Cycling-off errors result from the fact that, except with Proportional Assist Ventilation, current ventilator modes do not include any provision that links the end of ventilator cycle to end of patient's inspiratory effort. In the most common form of assisted ventilation, volume-cycled ventilation, the user sets the duration of the inflation cycle without knowledge of the duration of patient's inspiratory effort. Thus, any agreement between the ends of ventilator and patient inspiratory phases is coincidental. With the second most common form, pressure support ventilation, the inflation phase ends when inspiratory flow decreases below a specified value. Although the time at which this threshold is reached is, to some extent, related to patient effort, it is to the largest extent related to the values of passive resistance and elastance of the patient. In patients in whom the product [resistance/elastance], otherwise known as respiratory time constant, is high, the ventilator cycle may extend well beyond patient effort, while in those with a low time constant the cycle may end before the end of patient's effort (Younes M (1993) Patient-ventilator interaction with pressure-assisted modalities of ventilatory support. Seminars in Respiratory Medicine 14:299-322; Yamada Y, Du H L (2000) Analysis of the mechanisms of expiratory asynchrony in pressure support ventilation: a mathematical approach. J Appl Physiol 88:2143-2150). The present invention concerns methods and devices to insure that the end of the ventilator cycle does not deviate substantially from the end of patient's effort. This is achieved by insuring that the duration of the ventilator's inflation phase is a physiologic fraction (0.25-0.50) of the patient's respiratory cycle duration (patient TTOT). In this fashion enough time is available for lung emptying during the patient's expiratory phase. By extension, this also reduces dynamic hyperinflation at the onset of patient efforts, thereby also minimizing trigger delays and further improving synchrony.
In PCT/CA03/00976, filed Jun. 27, 2003, (WO 2004/002561), from which this application claims priority, I described an approach to generate a semi-quantitative estimate of the pressure waveform generated by the patient's respiratory muscles. This waveform can be used to identify the onset and end of patient's efforts. According to the aforementioned invention, the end of patient's inspiratory effort, detected by said invention, can be used to cycle off the ventilator, thereby insuring synchrony between the ends of ventilator and patient's inspiratory phases. There is, however, one potential complication to this approach. At times, end of patient effort occurs soon after ventilator triggering. This is because inspiratory muscle activity can be inhibited if inspiratory flow is high, and the ventilator frequently delivers excessive flow soon after triggering. Thus, this approach may result in medically unacceptable inflation times. It was recommended that a back-up procedure be included to insure that the duration of inflation phase is physiologically appropriate. A number of approaches to insure a physiologically appropriate duration of the inflation phase were proposed. These were in part derived from a separate application concerned specifically with methods to synchronize end of ventilator cycle with end of patient effort that do not require knowledge of when said patient efforts end (U.S. provisional Application 60/454,533, Mar. 14, 2003, from which this application claims priority). The current application describes rationale and implementation of said methods in detail and, additionally, introduces other approaches described in the Mar. 14, 2003 U.S. Provisional application (60/454,533) and not referred to in PCT/CA03/00976. The following is the rationale and method for ensuring that the duration of the inflation phase remains within physiologic limits.
In spontaneously breathing subjects and patients, the duration of the inspiratory phase (TI) ranges between 25% and 50% of respiratory cycle duration (TTOT). In studies by the inventor using proportional assist ventilation (PAV), with which the duration of the ventilator's inflation phase mirrors the patient's own TI, the ratio of TI to TTOT (TI/TTOT ratio) was also found to be between 0.25 and 0.5. Therefore, one approach to insure that the duration of the inflation phase is within the physiologic range is to constrain the duration of the inflation phase to be between 0.25 and 0.50 of the total cycle duration of patient's own efforts (to be distinguished from duration of ventilator cycles). Implementation of this procedure requires knowledge of the true respiratory rate of the patient (as opposed to ventilator rate). The inventor, in association with his students and technicians, described a method for visually determining true patient rate by identifying visually distinctive patterns in the waveforms of respiratory flow and airway pressure (Giannouli et al, American Journal of Respiratory and Critical Care Medicine, vol 159, pages 1716-1725, 1999). According to this approach, true patient rate is the sum of ventilator rate, the number of ineffective efforts occurring during the ventilator's exhalation phase (arrows marked “c”, FIG. 1) and the number of additional efforts occurring during inflations triggered by an earlier effort (arrows marked “b”, FIG. 1). In PCT/CA03/00976 ventilator cycles triggered by patient (arrows “a”, FIG. 1) as well as ineffective efforts occurring during exhalation (arrows “b”, FIG. 1) are to be automatically detected from the new composite signal generated from the flow, Paw and volume signals. In the present invention, I describe another approach for identifying ineffective efforts. An approach was described in PCT/CA03/00976 to identify additional efforts occurring during the inflation phase (arrows “c”, FIG. 1). This approach is retained here with minor modifications.
As indicated in U.S. Provisional application 60/454,533, and also in PCT/CA03/00976, once the true respiratory rate of patient is known, it becomes possible to calculate the real duration of respiratory cycles of the patient (TTOT=60/respiratory rate) and determine the range of inflation times consistent with a physiologic TI/TTOT. For example, if patient's rate is 30/min, TTOT is 2.0 seconds and the physiological range for the inflation phase is 0.5-1.0 second reflecting a TI/TTOT range of 0.25 to 0.50. The desirable duration of the ventilator's inflation phase is then determined by multiplying patient TTOT by a user selected physiologic TI/TTOT ratio or a suitable default value (e.g. 0.4). The ventilator's inflation phase can then be made to cycle off after said desirable duration.
There are a number of ways by which the duration of the ventilator's inflation phase can be made to correspond to desirable TI. One approach, discussed in U.S. Provisional application 60/454,533 and also proposed in PCT/CA03/00976, is to terminate the inflation phase at the specified desirable duration following onset of inspiratory effort or following the time of ventilator triggering. With this approach ventilator inflation varies strictly with average respiratory rate discerned from a number of elapsed breaths. There is no provision, therefore, for accommodating breath-by-breath changes in duration of inspiratory effort since the desirable duration is predetermined before the effort begins (based on an average result obtained from a number of elapsed breaths). Another approach, particularly suited for pressure support ventilation, is to retain the usual criterion for terminating the inflation phase, namely when inspiratory flow reaches a specified threshold, but flow threshold is adjusted to produce the desired TI. This would permit breath-by-breath changes in patient's TI to influence ventilator TI in current breaths but ventilator TI would, on average, correspond to desirable TI. This general approach was proposed in U.S. Provisional application 60/454,533. In PCT/CA03/00976 I proposed that this general approach be implemented by measuring the flow occurring at the desirable TI in a number of elapsed breaths. This would then become the flow threshold for terminating the inflation phase in prospective (i.e. current) breaths. An alternative approach proposed in U.S. Provisional application 60/454,533 (but not in PCT/CA03/00976) is to measure actual ventilator TI in a number of elapsed breaths. This actual value is compared with desirable TI with the difference (i.e. actual TI−desirable TI) representing an error signal that can be used for closed-loop control of the flow threshold for cycling off, using any of a number of closed-loop control approaches. Alternatively, the error signal can be the difference between actual TI/TTOT (i.e. actual TI/patient TTOT) and desirable TI/TTOT.
In my experience, patient's respiratory rate often changes substantially from time to time. An essential feature of this invention is, therefore, the provision for automatic means to monitor patient respiratory rate and to update the relevant values (e.g. desirable TI, actual TI, TI error . . . etc) at frequent intervals.
With current methods of assisted ventilation tidal volume is directly related to the duration of the inflation phase. Changes in the duration of the inflation phase produced by the methods of the current invention are, therefore, expected to result in corresponding changes in tidal volume. In another aspect of the current invention provision is made to partially or completely offset the resulting changes in tidal volume by concomitantly increasing inspiratory flow (in the case of volume-cycled ventilation) or the support pressure (in the case of pressure support or assist/pressure control ventilation) when the duration of the inflation phase is decreased, and vice versa.